From the spawn, turn around completely then go to that direction until you see a hole in the ground. No need digging, follow sidestep then after awhile you reach to the dungeon. After entry to the dungeon is lava pool on your left if you go to that direction until you see typical dungeon blocks on your right and a small entrance that is entrance to the tunnel system. This tunnel system isn’t complex, actually it quite easy to imagine this tunnel system in your brain. After a few minutes explore you will find diamond blocks soon.